A surface position detecting apparatus of an oblique incidence type disclosed in Japanese Patent Application Laid-open No. 2001-296105 (Patent Document 1) by the same applicant is known as a conventional surface position detecting apparatus suitable for projection exposure apparatus. In order to theoretically enhance the detection accuracy of the surface position of a detection target surface in this surface position detecting apparatus of the oblique incidence type, it is necessary to set an angle of incidence of a beam to the detection target surface large (or close to 90°). In this case, it is suggested that parallelogram prisms with a pair of internal reflection surfaces parallel to each other (which will be referred to hereinafter as “rhomboid prisms”) are disposed in respective optical paths of the projecting optical system and the condensing optical system to locate the projecting optical system and the condensing optical system away from the detection target surface (cf. FIG. 7 in Patent Document 1).    Patent Document 1: U.S. Pat. No. 6,897,462.